How can I change IntProperty default value stored within PropertyGroup?
Use a setter getter
Instead of setting the default of the property, emulate a property with a getter setter that retrieves the custom property when set, else some default we have set.
Internal &quot;get/set&quot; function of property?
As an example have added another intproperty in same prop group as a means to adjust the default value without any re-registering trickery.. which can turn ugly and strip the RNA if it gets out of sync.

Demo gif, changing setting of default is matched by myint till set
While myint has not been set it is given the default value. Once it is set it uses the value set.
import bpy
from bpy.props import PointerProperty
from bpy.types import PropertyGroup, Panel

class AAA_PT_Demo(Panel):
    bl_space_type = &quot;VIEW_3D&quot;
    bl_context = &quot;objectmode&quot;
    bl_region_type = &quot;UI&quot;
    bl_label = &quot;My Demo&quot;
    bl_category = &quot;Demo&quot;

    def draw(self, context):
        settings = context.scene.mysettings
        layout = self.layout
        layout.prop(settings, 'default')
        layout.prop(settings, 'myint')


class MySettings(PropertyGroup):
    def get_myint(self):
        return self.get(&quot;myint&quot;, self.default)
    
    def set_myint(self, value):
        self[&quot;myint&quot;] = value
        
        
    default : bpy.props.IntProperty(
        name = &quot;My Int Default&quot;,
        step = 5,
        default = 44
        )    
    myint : bpy.props.IntProperty(
        name = &quot;My Int&quot;,
        step = 5,
        get=get_myint,
        set=set_myint,
        )

classes = (MySettings,
           AAA_PT_Demo,
           )
           
def register():
    for cls in classes:
        bpy.utils.register_class(cls)
    bpy.types.Scene.mysettings = PointerProperty(type=MySettings)

def unregister():
    for cls in reversed(classes):
        bpy.utils.unregister_class(cls)
    del bpy.types.Scene.mysettings

if __name__ == &quot;__main__&quot;:
    register()

Other possiblilties are storing the default properties on another &quot;toolsettings&quot; prop group, or instead of using an annotated property def, simply declare a class variable default = 44 in the prop group and grab it from the class.